de Bruyne U.S. Pat. No. 4,465,377 discloses apparatus for stirring liquid material, specifically culture medium. As therein noted, liquid culture medium provides for the growth of cells from nutrients which are contained in the medium. The stirring action which is to be imparted to such liquid culture medium is not a violent stirring, but is a gentle stirring, having as goals not only the stirring of the liquid culture medium and exposure of all cells to the gas above the liquid surface, but the avoidance of damage to the cells, such as would be occasioned by violent agitation, or by crushing of cells, as between two relatively moving parts. In that patent, there is disclosed a flask, with a buoyant, magnetically actuable stirrer within the flask, and an guide rod or shaft extending generally axially of the flask from top to bottom, the stirrer being movable along the guide rod or shaft as the liquid level is varied within the flask. The floating stirrer in some embodiments comprises a magnetic element, and on the exterior of the flask there is provided means for generating a rotating magnetic field to cause rotation of the magnetic element and the buoyant stirrer of which it forms a part. In some embodiments, a guide rod, supported in a lid of the flask, extends to the bottom of the flask, and passes through the buoyant stirrer. In another embodiment, a rotatable drive shaft is supported from the lid, and has a floating stirrer drivingly connected to it for rotation with it, the driving connection permitting movement of the stirrer along the drive shaft as the liquid level changes.
The above-noted patent makes reference to a number of preceding patents, which are incorporated herein by reference.
The structure disclosed in de Bruyne U.S. Pat. No. 4,465,377, while generally providing for satisfactory operation, has been found to be subject to improvement. For example, although the structure provided in the noted patent is constructed so that there is reduced danger of the crushing of cells as between relatively movable elements, the construction has not proven as free of risk as is desirable. In that construction, for example, the engagement between the structure of the floating stirrer and the guide rod or shaft was above the level of the liquid. While this construction minimized the risk of cell damage by liquid entering between the rod or shaft, on the one hand, and the floating stirrer on the other hand, the risk of crushing the cells was not completely eliminated.
A further improvement which is desirable is in connection with the flow of the liquid in the flask. The rotation of a buoyant stirrer effects approximately a "Thomson Secondary flow," causing liquid to be directed generally outwardly by the stirrer, the liquid then striking the walls of the flask and travelling downwardly towards the bottom and then spiralling upwardly along and about the axis of the flask to the stirrer, thereby providing for circulation and stirring of substantially all of the liquid culture medium in the flask. This is important since it is highly desireable that all of the cells, or as a high proportion of the cells as is possible, be exposed to the gaseous atmosphere which exists at the gas-liquid interface in the flask.